The present invention generally relates to using metamaterial structures in high-power microwave vacuum electron devices and more particularly, it relates to using the metal-thin-wire medium as an electron emitter in multi-cavity magnetrons. The metal wires are thin in relation to the wavelength of the magnetron operation only. The actual thickness of the metal wires may be as large as necessary for handling the appropriate thermal loads within the multi-cavity magnetrons.
The multi-cavity magnetron may be considered as a cylindrical magnetron diode with the cathode emitting electrons and the anode consisting of a number of resonant cavities. The cavities slow down the circumferential component of the electromagnetic wave, which azimuthally travels between the cathode and the anode. The traveling electromagnetic wave is associated with the radio frequency (rf) electric field induced within the magnetron. The external DC electric field E0 in the magnetron diode is directed radially from the anode to the cathode. The external DC magnetic field H0 is directed axially. The crossed external DC electric and magnetic fields (E0×H0) force the electrons emitted from the cathode to drift in the azimuthal direction. This drift forms the electron space-charge cloud surrounding the cathode. The multi-cavity magnetron starts to generate (induce) the rf electric field and produce microwaves (output power) when the azimuthal drift velocity of the emitted electron is approximately equal to the phase velocity of the circumferential component of the electromagnetic wave traveling in the same azimuthal direction.
The operating mode of the multi-cavity magnetron is the transverse-electrical-like (TE-like) mode of the magnetron resonant cavity. The resonant cavity is formed by the cathode and the anode of the magnetron. The frequency of the magnetron operation is determined by: (i) the geometry of the resonant cavity; (ii) the relation between the external DC electric and magnetic fields; and (iii) the amount of the space charge accumulated within the electron flow rotating between the cathode and the anode. The spatial distribution of the induced rf electromagnetic field within the resonant cavity is characterized by the TE-like mode pattern. This pattern is characterized by the circumferential E1Φ and the radial E1ρ components of the electric field and the axial H1z component of the magnetic field. The number of the phase variations of the induced rf electric field in the azimuthal direction is determined by the operating frequency of the magnetron (mode of the magnetron operation) and the anode geometry (number of cavities). The number of the induced rf electric field phase variations along the radius and the axis of the resonant system are minimized. This allows the magnetron to operate in the lower-frequency TE-like mode and prevents both the axial and the radial modes competing.
Cathode priming of multi-cavity magnetrons results in faster start-up times, locking of the magnetron oscillations into the desired magnetron operating mode, and increasing the total radiated microwave power/energy. Among earlier methods of cathode priming are: (i) the artificial selection of the emitting regions on a surface of a solid cylindrical cathode (the PAL cathode of the University of Michigan); (ii) change of a geometrical shape of the solid cylindrical cathode (the shaped cathode of the U.S. Pat. No. 7,245,082 B1, Jul. 17, 2007), FIG. 6; and (iii) removal of longitudinal strips from a thin-walled tubular cathode (the transparent cathode of U.S. Pat. No. 7,696,696 B2, Apr. 13, 2010), FIG. 7. A new method of cathode priming of multi-cavity magnetrons using metamaterial structure as a unique cathode design is presented here resulting in improved output characteristics.